This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-274878, filed Sep. 28, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a multi-beam optical scanning device capable of being utilized for a multi-drum type color printer, a multi-drum type color copier, a high-seed laser printer, a digital copier and the like for scanning a plurality of beams.
In an image forming apparatus such as the multi-drum type color printer or the multi-drum type color copier, for example, a plurality of image forming units corresponding to separated color components, and an optical scanning device (laser light exposure apparatus) for supplying, to the image forming units, image data, i.e., a plurality of laser beams corresponding to the color components are used.
Two types of this kind of image forming apparatuses are known, i.e., a type in which a plurality of optical scanning devices corresponding to the image forming units are disposed, and a type in which a multi-beam optical scanning device capable of supplying a plurality of laser beams is disposed.
The optical scanning device includes a plurality of semiconductor laser elements (laser diodes) as light sources, a first lens group (forwardly deflecting optical system) for reducing a sectional beam diameter of each of a plurality of laser beams ejected from each of the laser diodes, a polygonal mirror unit for continuously reflecting the plurality of laser beams stopped down by the first lens group in a direction perpendicular to a direction into which a photosensitive member is transferred, and a second lens group (post deflecting optical system) for forming an image of the laser beams deflected by the polygonal mirror unit onto a predetermined position of the photosensitive member. In many cases, a direction in which the laser beams are deflected by the polygonal mirror unit is indicated as a sub-scanning direction perpendicular to the direction into which the photosensitive member is transferred, i.e., a main-scanning direction.
The plurality of laser beams which has passed through the first lens group are continuously reflected (deflected) by a reflecting surface of a polygonal mirror which is rotated at a predetermined rotation number, and are deflected to the main-scanning direction. Each deflected laser beam is formed as an image on a predetermined position of the photosensitive member.
The second lens group provides the laser beam scanned by the polygonal mirror with substantially the same sectional beam diameter on the photosensitive member, and provides the laser beam with different focusing degree whenever the beam is reflected by the polygonal mirror.
If a plastic lens having a surface-inclination correcting function is used in the post deflecting optical system (second lens group) of the above-described optical scanning device, it is necessary to avoid influence of temperature and moisture. To avoid this, Jpn. Pat. Appln. KOKAI Publication No. 9-189872 and the like proposed a hybrid cylinder lens having a resin cylinder lens which has a negative power in the sub-scanning direction but does not have a power in the main-scanning direction, and a glass cylinder lens having a positive power in the sub-scanning direction.
when the hybrid cylinder lens including the plastic lens having the surface-inclination correcting function is used in the post deflecting optical system as described above, since the a radius of curvature is constant in the sub-scanning direction, it is difficult to set spherical aberration and coma aberration to predetermined values to cancel the spherical aberration and the coma aberration generated in the post deflecting optical system. This deteriorates RMS-OPD on an image surface, and there is a problem that a flare in the sub-scanning direction and the like are increased.
Further, the conventional optical scanning device only has the same function as a flat plate in the main-scanning direction, and the aberration and the like can not be provided positively. Therefore, it is difficult to correct the spherical aberration generated in the post deflecting optical system. This deteriorates RMS-OPD on an image surface, and there is a problem that a flare in the main-scanning direction and the like are increased.
Further, in the case of the hybrid lens, since it is necessary that a concave surface of the plastic lens and a convex surface of the glass cylinder lens having the positive power in the sub-scanning direction must have the same curvatures, it is difficult to optimize both the coma aberration and the spherical aberration.
If surfaces having curvatures of the glass lens and the plastic lens are connected to each other, automatic aligning mechanism functions, and there is a merit that parallelism of the buses can be maintained, but on the other hand, if the shape of the plastic lens loses and non-contact surface is created between the plastic lens and the glass lens, the contacting surface has the curvature of glass and the non-contact surface has the curvature of plastic and as a result, the lens has two focal points. Therefore, there is a problem that a flare is generated in the image surface and the beam diameter is increased.
It is an object of the present invention to provide an optical scanning device capable of preventing a flare from being generated and a beam diameter from increasing by using a post deflecting optical system including a plastic lens.
The present invention provides an optical scanning device comprising: a light source; forwardly deflecting optical set including a first lens for providing light beams from the light source with a predetermined characteristic, and a second lens for converging the light beams from the first lens in a first direction; a polygonal mirror unit for deflecting the light beams from the forwardly deflecting optical set into a second direction substantially perpendicular to the first direction; a third lens for forming the light beams deflected by the polygonal mirror unit as an image onto a predetermined image surface at substantially equal speed; wherein the second lens includes a resin lens and a glass cylinder lens made of glass having a positive power in the first direction and wherein the resin lens of the second lens having a surface whose radius of curvature in the first direction is varied along the first direction.
Further, the present invention provides an optical scanning device comprising: a light source; forwardly deflecting optical set including a first lens for providing light beams from the light source with a predetermined characteristic, and a second lens for converging the light beams from the first lens in a first direction; a polygonal mirror unit for deflecting the light beams from the forwardly deflecting optical set into a second direction substantially perpendicular to the first direction; a third lens for forming the light beams deflected by the polygonal mirror unit as an image onto a predetermined image surface at substantially equal speed; wherein the third lens having a positive power in the second direction, and the second lens-having a power in the second direction.
Furthermore, the present invention provides an optical scanning device comprising: a light source; forwardly deflecting optical Bet including a first lens for providing light beams from the light source with a predetermined characteristic, and a second lens for converging the light beams from the first lens in a first direction; a polygonal mirror unit for deflecting the light beams from the forwardly deflecting optical set into a second direction substantially perpendicular to the first direction; a third lens for forming the light beams deflected by the polygonal mirror unit as an image onto a predetermined image surface at substantially equal speed; wherein the second lens includes a resin lens including a surface having a negative power in the first direction, and a glass lens including one convex surface having a positive power in the first direction, the resin lens of the second includes a projection which abuts in a direction of the convex surface of the glass lens, and wherein the projection of the resin lens and the glass cylinder lens contact with each other.
Still further, the present invention provides an optical scanning device comprising: a light source; forwardly deflecting optical set including a first lens for providing light beams from the light source with a predetermined characteristic, and a second lens for converging the light beams from the first lens in a first direction; a polygonal mirror unit for deflecting the light beams from the forwardly deflecting optical set into a second direction substantially perpendicular to the first direction; a third lens for forming the light beams deflected by the polygonal mirror unit as an image onto a predetermined image surface at substantially equal speed; wherein the second lens includes a resin lens including a surface having a negative power in the first direction, and a glass lens including one convex surface having a positive power in the first direction, a deformable sheet having a substantially constant thickness is provided between the resin lens and the glass cylinder lens, and each of the resin lens and the glass cylinder lens has a space portion when both the lenses come into contact with each other.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.